10 year-old boy. For 3 months he complains of moderate pain in the lower leg region. Pain occurred in variable intensity during the day, but particularly at night. Treatment so far: immobilization.

Pathomorphology or Pathophysiology of this disease :

Osteoid osteomas are, beside fibrotic cortical defects and osteomas, the third most frequent benign bone change of youth. It appears more frequently in boys than in girls. Usually, it is localized in the proximal femur, but it can also be frequently found in the tibia and in the spinal column. Also, all other bones (phalanges) can be afflicted. Nightly bone pain is typical as well as the prompt diminishing of pain when treated with aspirin.

Centrally one finds the nest; the nest cavity is often hypervascularized. Surrounding reaction: Hyperostosis with often distinct asymmetrical cortical thickening.

Radiological findings:

X-ray 1: one-sided cortical thickening of the tibia. One circumscribed, lytic change cann not be proven in this x-ray image.

MRI 1: T1 with fat saturation, transversal: in the dorsolateral area of the tibia within the cortical hyperostosis, a little, non-enhancing nidus and a border with contrast-uptake is seen.